An airplane includes landing gear to facilitate takeoff, landing and taxi. The landing gear of some aircraft includes a shock strut that is pivotally attached to a truck beam at a distal or lower end thereof. The truck beam includes two or more axles upon which tires are mounted. In this regard, the truck beam may include a forward axle positioned forward of the shock strut and an aft axle positioned aft of the shock strut. Upon takeoff, an airplane having a conventional landing gear with forward and aft axles will pivot about the pin that attaches the truck beam to the shock strut such that all of the landing gear tires have an equal load distribution.
In order to provide additional ground clearance for rotation of the aircraft during takeoff, semi-levered landing gear mechanisms have been developed. A semi-levered landing gear fixedly positions the shock strut and the forward end of the truck beam during takeoff such that the forward axle is in a raised position relative to the aft axle when the airplane has left the ground. As such, the aircraft pivots about the aft axle, rather than the pin that pivotally connects the truck beam to the shock strut provided that the extend pressure of the shock strut has been increased sufficiently. By rotating about the aft axle, the landing gear height is effectively increased so as to provide additional ground clearance for rotation of the aircraft during takeoff. As a result, the takeoff field length (TOFL) of the aircraft may be reduced, the thrust required of the engines may be reduced or the weight carried by the aircraft may be increased while maintaining the same takeoff field length.
In order to provide for rotation of the aircraft about the aft axle during takeoff, a semi-levered landing gear locks the truck beam in a “toes-up” attitude such that the tires mounted upon the aft axle support the aircraft, while the tires mounted upon the forward axle are raised above the surface of the runway. Following takeoff, the landing gear is generally stowed in a wheel well or the like. In order to fit within a conventional wheel well, the landing gear must typically be unlocked and the truck beam repositioned in a “stowed” attitude prior to retracting the landing gear into the wheel well. Thereafter, during landing, the landing gear is lowered and the truck beam is repositioned such that all of the wheels, including both those on the forward axle and the aft axle, equally bear the weight of the aircraft. Typically, the locking and unlocking of a semi-levered gear system and the resulting repositioning of the truck beam relative to the shock strut occurs without input from the pilot or the flight control system.
One type of semi-levered landing gear includes a locking hydraulic strut to lock the truck beam in the desired orientation for takeoff. The locking hydraulic strut is essentially a locking actuator, but has a number of additional chambers and an internal floating piston. See, for example, U.S. Pat. No. 6,345,564. While a semi-levered landing gear having a locking hydraulic strut is suitable for some aircraft, the landing gear of other aircraft may not have sufficient clearance or room for the hydraulic strut to be positioned between the shock strut and the truck beam in an efficient manner. In addition, the hydraulic strut disadvantageously adds to the cost and complexity of the landing gear.
Another semi-levered landing gear utilizes a mechanical linkage to lock the truck beam during takeoff, but requires a separate mechanical linkage, termed a shrink-link, to reposition the shock strut for retraction into the wheel well. The requirement for a shrink-link disadvantageously increases the complexity, expense and weight of the resulting semi-levered landing gear.
Accordingly, it would be desirable to provide an improved semi-levered landing gear that reliably positions the truck beam in a toes-up attitude during takeoff and then repositions the truck beam in a “stowed” attitude following takeoff for stowage in the wheel well. In particular, it would be desirable to provide a semi-levered landing gear that is both weight and cost efficient and that is not overly complex, while still satisfying the various operational requirements of the semi-levered landing gear.